Imperfections key performance of new light-detecting crystals

Imperfections key performance of new light-detecting crystals

By seeing an old problem in a new light, UNL engineers have developed a simple and low-cost method for building detectors that pluck slivers of visible, infrared and ultraviolet wavelengths from the electromagnetic spectrum.

The researchers engineered a series of light-detecting crystals that can register especially narrow bands of wavelengths, tailoring the location and width of these bands by adjusting the crystals’ chemical composition.

A recipe that yielded an orange-colored crystal, for instance, responded only to forms reflecting those wavelengths – objects the brain perceives as orange – while suppressing the “noise” of all other waves.

“Basically, you can just choose the material that matches up with the application you’re interested in – usually in the visible and infrared range – and create whatever window (of wavelengths) you need,” said Jinsong Huang, a Susan J. Rosowski associate professor of mechanical and materials engineering.

The new process could greatly simplify the design and manufacturing of these narrowband photodetectors, Huang said, while allowing users to select bands from a greater proportion of the electromagnetic spectrum than offered by previous designs.

In various forms, narrowband photodetectors have been used to capture fluorescent imagery for biomedical research and track weapon caches during military reconnaissance, among other applications.

“This kind of device often includes a combination of band-pass filters that cost hundreds of dollars and a detector that costs thousands or more,” Huang said. “The detectors that we just reported basically combine the functions of both at a cost tens of times lower.”

As reported in the journal Nature Photonics, the team’s crystals can register wavelength bands narrower than 20 nanometers, roughly half the length of a typical virus. The crystals manage this precision thanks partly to microscopic blemishes that appear on their surface – the same defects that the team has overcome in boosting the efficiency of solar cells featuring similar atomic frameworks.

These natural defects can impede solar cell efficiency by trapping and recombining negatively charged electrons with their positively charged “holes,” which ideally produce electric current by migrating in opposite directions following their separation by photon-carrying sunlight.

Yet this problem became an advantage when the researchers set out to build crystals that register only a small fraction of the electromagnetic spectrum, which runs from relatively long radio waves to ultra-short Gamma rays. Though each crystal absorbs the entirety of the spectrum, its defects continue to trap the electrons and holes jarred loose by wavelengths that barely penetrate its surface.

More deeply absorbed wavelengths, however, can separate these charges far enough below the surface to prevent their recombination and allow their escape toward electrodes at each end of a photodetector. The device then reads and outputs only this narrow band of wavelengths, effectively ignoring any that reside outside of that range.

To test the importance of the surface defects, Huang and his colleagues compared the performance of an unwashed crystal with one cleansed in an organic liquid.

“Once we washed the crystals, the defects disappeared,” Huang said. “This widened the range of wavelengths they were responding to, which is the opposite of what you want for some applications.

“So you actually want these crystals to be kind of dirty on the surface. That actually helps make them work.”